Study of polarization-dependent coupling in optical waveguide directional couplers by the effective-index method with built-in perturbation correction

Polarization dependence is a major factor of consideration in the design of optical waveguide devices for application in optical fiber transmission systems. In this paper, we apply the recently proposed effective-index method with built-in perturbation correction to the study of the polarization dependence in the coupling coefficient of a directional coupler that consists of two parallel rectangular-core waveguides. Numerical results are found to agree with those calculated by the vector version of the Marcatili method with perturbation correction. With numerical illustrations, we show that the polarization dependence in the splitting ratio of a directional coupler can be eliminated by a proper choice of the physical parameters of the coupler. Our results provide useful guidelines for the design of polarization-insensitive integrated-optic directional couplers.

I. Introduction

Optical waveguide directional couplers are the building blocks of many integrated-optic devices, such as signal dividers, wavelength-division multiplexers, add/drop multiplexers, and modulators/switches based on Mach–Zehnder interferometers. The behavior of a directional coupler in the form of two parallel identical optical waveguides is characterized by a coupling coefficient, which is a measure of the difference in the propagation constant between the symmetric and antisymmetric modes of the composite waveguide structure [1]. In general, the coupling coefficient and, hence, the splitting ratio of a coupler, depends on the polarization state of light. This can give rise to polarization noise when light is launched into the coupler from a single-mode fiber. It is of practical interest to search for designs of directional couplers that have polarization-independent coupling coefficients. Fig. 1. Cross section of a directional coupler consisting of two parallel rectangular-core waveguides.